On a laboratory scale silicon nitride based ceramics containing sintering aids such as MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, ZrO.sub.2, HfO.sub.2, CeO.sub.2 and La.sub.2 O.sub.3 have been sintered to densities above 98% of theoretical density. When the sintering process has been upscaled for sintering large quantities of injection molded, isostatically pressed, or slip cast components of silicon nitride based compositions such as (Si.sub.3 N.sub.4 +6 wt% Y.sub.2 O.sub.3 +2 wt% Al.sub.2 O.sub.3), final sintered densities often fall below the desired 98% level.
It has been noted that the larger the number of silicon nitride parts sintered simultaneously in a closed sintering container, the greater the decrease in density. Studies of this effect have also demonstrated that parts placed towards the middle of the container tend to have both lower sintered density and lower weight loss during the sintering process (weight losses usually range from 0.5 to 2 w/o). Also noted has been a slight color change on the component surfaces where two components are physically close but not touching. These observations have led to the theory that during the sintering process gaseous species are generated from the parts which, if allowed to collect in the closed container, tend to inhibit sintering. The larger the amount of silicon nitride being sintered in a given container volume, the greater the collection of the sintering inhibitor and the lower the resultant densities. An ideal solution would be to use an open container instead of a closed one but this leads to accelerated Si.sub.3 N.sub.4 decomposition.